Method and system of recovering the heat wasted from the steam boilers continuous blow down to preheat the boiler combustion air

ABSTRACT

Heat from continuous blow down from a steam boiler is used to preheat boiler combustion air. Blow down from the steam boiler is fed through a flash tank and a 3-way valve to a liquid heat exchanger to heat a circulating liquid. The hot circulating liquid is fed to a preheat coil to heat combustion air for a steam boiler. A 3-way valve located between the flash tank and heat exchanger controls the flow of blow down to the heat exchanger. A temperature sensor which monitors the temperature of the circulating liquid controls the 3-way valve and a circulating pump moves the liquid around a closed loop. Motorized or variable frequency drive damper control the flow of air to the preheat coil and to the combustion air blower of the steam boiler.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the recovery of waste heat from a steam boiler's continuous blow down and more specifically to a method and system for using the recovered heat to preheat boiler combustion air.

2. Description of Related Art

In these days of rising fuel costs due to inflation, scarcity of fuel supplies, and rising fuel costs, it is desirable to provide a more efficient heating system which can maximize the quantity heat delivered by a heater for each unit of fuel consumed. Currently, many steam heating systems are inefficient which results in increased fuel usage and the increase cost associated with using additional fuel. Various methods have been proposed in an attempt to reduce the cost of operating heating systems by extracting greater amount of heat energy from the fuel used.

For example, U.S. Pat. No. 4,251,028 discloses using heat retrieved from flue gas passing through the stack to improve the efficiency of the boiler by increasing the supply of domestic hot water.

U.S. Pat. No. 3,022,985 discloses recovering blow down steam heat which is captured by means of a spray during a short blow down period to transform such heat into usable boiler feed water heat during a subsequent cycle.

U.S. Pat. No. 1,818,483 discloses using the heat from blow off water having a temperature of approximately 212 degrees F. to heat cold feed water in a heat exchanger where the temperature of the feed water being heated is only lower than the hot blow off water.

U.S. Pat. No. 6,889,910 discloses a system for controlling a combustion environment to reduce fuel cost by providing a first heating unit for adjusting the temperature of the fuel fed to a burner, and a second heating unit for adjusting combustion air temperature prior to ignition of the heating element.

U.S. Pat. No. 1,565,304 discloses an economizer for steam boilers where hot gas from the stack are used to heat boiler feed water. In an embodiment, the feed water flow is from the bottom to the top of the economizer as the hot gas flow is from the top to the bottom.

U.S. Pat. No. 4,738,226 discloses a steam boiler having a rear flue having a plurality of gas passages where each gas passage may have a gas at a different temperature. Each passage has heat transfer means and dampers which are use to control the flow of gas through the passage. Down stream of the dampers the passages are combined where the gases mix. The temperature of the gas in each passage is controlled by the dampers to keep the temperature of the mixed gases substantially constant.

What is needed is a new method and system of using waste heat to reduce the cost of operating steam boilers by extracting greater amounts of heat energy from the fuel used.

SUMMARY OF THE INVENTION

Waste heat from continuous blow down from a steam boiler is used to preheat boiler combustion air to improve the operating efficiency of the steam boiler.

In an embodiment, blow down from the steam boiler is fed to a flash tank and then to a heat exchanger via a 3-way valve where the hot blow down is used to heat a circulating solution of ethylene glycol and water. The 3-way valve controls the flow of blow down to the heat exchanger. A circulating pump moves the heated glycol/water solution around a closed loop which includes a liquid-to-air heat exchanger (preheat coil) where heat from the solution is used to heat combustion air before it reaches the steam boiler. The temperature of the circulating glycol/water solution is monitored by a temperature sensor which controls the 3-way valve to prevent the glycol/water solution from boiling. A motorized damper located downstream of the preheat coil controls the amount of combustion air passing thru the combustion air blower and the preheat coil. This damper can be substituted with a variable frequency drive. The other motorized damper located upstream of the preheat coil is for freeze protection. If the boiler is off, the combustion air blower is off if a variable frequency drive is used the outside cold air could migrate to the boiler and freeze the water. The freeze protection damper will be closed when the boiler and the combustion air blower are off.

The foregoing has outlined, rather broadly, the preferred feature of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention and that such other structures do not depart from the spirit and scope of the invention in its broadest form.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention will become more fully apparent from the following detailed description, the appended claim, and the accompanying drawings.

FIG. 1 is a schematic diagram showing a system for recovering heat from boiler continuous blow down to preheat boiler combustion air in accordance with the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to recovering waste heat from a steam boiler's continuous blow down and using the recovered heat to preheat the boiler's combustion air. Blow down from the steam boiler is fed to a flash tank and then to a heat exchanger, thru a 3-way valve. In the heat exchanger the hot blow down is used to heat a circulating solution of ethylene glycol and water. The 3-way valve controls the flow of blow down to the heat exchanger.

A circulating pump moves the heated glycol/water solution around a closed loop which includes a liquid-to-air heat exchanger (preheat coil) where heat from the solution is used to heat combustion air before it reaches the steam boiler. The temperature of the circulating glycol/water solution is monitored by a temperature sensor which controls the 3-way valve to prevent the glycol/water solution from boiling.

A motorized damper located downstream of the preheat coil controls the amount of combustion air passing thru the combustion air blower and the preheat coil. This damper can be substituted with a variable frequency drive. The other motorized damper located upstream of the preheat coil is for freeze protection. If the boiler is off, the combustion air blower is off and if a variable frequency drive is used instead of the combustion air dumper, the outside cold air could migrate to the boiler and freeze the water. The freeze protection damper will be closed when the boiler and the combustion air blower are off.

Referring to FIG. 1, there is shown a system for recovering heat from a boiler's continuous blow down to preheat boiler combustion air. A steam boiler 1 is coupled to feed continuous blow down to flash tank 2. Blow down from flash tank 2 is fed through a 3-way control valve, 3, to heat exchanger 4.

Blow down controls the concentration of deposits in boiler water. In this invention blow down heat is recovered and reused to heat combustion air that is fed to a steam boiler. Continuous blow down uses a probe which constantly monitors the conductivity of the boiler water and adjusts the blow down rate to maintain a desired water chemistry.

Blow down which is received directly from a steam boiler has the same temperature and pressure as the boiler water. Therefore, the heat in blow down is recovered with a flash tank and a heat exchanger.

Returning to FIG. 1, blow down from the boiler is sent thru the flash tank 2, to the heat exchanger 4. The temperature of the blow down leaving the flash tank is usually above 220° F. Blow down from the flash tank is fed thru a 3-way valve to a heat exchanger 4.

In the heat exchanger, heat from the blow down is transferred to a glycol/water solution which moves past a temperature sensor 5 to a preheat coil 7. When the temperature sensor 5 detects that the glycol/water solution is approaching its boiling point, a 3-way control valve, 3, automatically diverts incoming blow down from the flash tank to a drain 11. Blow down which has passed through heat exchanger is discharged through discharge pipe 12 to a drain which can be drain 11. The glycol/water solution is circulated through preheat coil 7 and heat exchanger 4 by circulator pump 6. At preheat coil 7 heat from the glycol/water solution is transferred to combustion air that is being moved to the boiler 1 by air blower 8. The Combustion air control motorized damper 9 which is located down stream of the combustion air blower 8 controls the flow of air through the preheat coil 7 to modulate the flow of heated combustion air fed to the combustion air burner. Thus, as the flow of heated air to the boiler is restricted, so is the flow of air through the preheat coil restricted. This change in the volume of air flowing through the preheat coil will cause a change in the temperature of the glycol/water solution. As the air flowing through the preheat coil decreases, the temperature of the glycol/water solution increases. The other motorized damper 10 located upstream of the preheat coil is for freeze protection. If the boiler is off, the combustion air blower is off and, if instead of a combustion air damper a variable frequency drive is used, the outside cold air could migrate to the boiler and freeze the water. To avoid this, the freeze protection damper will be closed when the boiler and the combustion air blower are off.

The combustion air that flows thru damper 10 can be from an indoor air intake 13, and outdoor air intake 14, or from both.

During operation, when the steam boiler 1 is on and the system is energized, the steam boiler 1 is operating, the combination air blower 8 is on, the circulation pump 6 is on, and the outside air motorized damper 10 is open to pass combustion air to the preheat coil 7. The 3-way control valve 3 operates to modulate the flow of blow down to heat exchanger 4. If, when the system is on, the temperature of the ethylene glycol/water solution rises to be within 20 degrees F. of its boiling point, the 3-way control valve 3 will operate to dump blow down from the flash tank down the drain. The temperature at which the ethylene glycol/water solution will boil is determined by the percentage of ethylene glycol that is in the solution because the higher the percentages of ethylene glycol the higher the boiling point. In operation, the actual temperature at which the 3-way control valve starts to dump the blow down is set by the operator.

During operation when the boiler is off, the circulating pump 6 will be off and the combustion air blower will be off. The 3-way control valve 3 will be operated to divert the entire blow down into the drain and the outside air motorized damper 10 will be closed.

Controls for operating the various components are integrated with boiler controls and/or the building automation system.

In the invention disclosed, heat from continuous blow down from a steam boiler is used to preheat the boiler combustion air to improve the operating efficiency of the steam boiler. Blow down from the steam boiler is fed to a flash tank and then fed through a 3-way valve to a heat exchanger where the hot blow down is used to heat a circulating solution of ethylene glycol and water. The heated glycol/water solution circulates through a preheat coil 7 where combustion air that is being fed to a steam boiled is heated. The preheat coil is a liquid-to-air heat exchanger. A 3-way valve which is located between the flash tank and heat exchanger controls the flow of blow down to the heat exchanger. A temperature sensor used to monitor the temperature of the glycol/water solution is coupled to control the 3-way valve, and a circulating pump is used to circulate the glycol/water solution in a closed loop. Motorized or variable frequency drive dampers are used to control the flow of air to the preheat coil and to the combustion chamber of the steam boiler.

It is to be understood that the sizes of the equipment here disclosed and the temperature settings that are used will depend upon the specific equipment used and its application.

The present invention materially contributes to increasing the utilization of waste energy from the boilers by preheating the combustion air with waste heat, saving fuel.

Based upon the foregoing, it will be apparent that there has been provided a new and useful method and system to use the waste heat from the boiler continuous blow down to preheat the boiler combustion air, which will save energy.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to the preferred embodiments, it will be understood that various omissions and substitutions and changes of the form and details of the apparatus illustrated and in the operation may be done by those skilled in the art, without departing from the spirit of the invention. 

1. A system for heating combustion air before the combustion air is received by a steam boiler comprising: a flash tank coupled to receive blow down from the steam boiler; a heat exchanger coupled to receive blow down from said flash tank, the said heat exchanger will recover the heat from blow down, and transfer the heat to a liquid and a circulating pump who moves the liquid between the heat exchanger and preheat coil, a preheat coil coupled to receive said circulating liquid and combustion air to transfer heat from the circulating liquid to the combustion air.
 2. The system of claim 1 wherein said heat exchanger is a liquid-to-liquid heat exchanger.
 3. The system of claim 2 further comprising: a first damper positioned to modulate the flow of combustion air from said preheat coil.
 4. The system of claim 3 wherein said first damper is located downstream of the combustion air blower.
 5. The system of claim 4 further comprising: a second damper located to modulate combustion air to the preheat coil.
 6. The system of claim 5 further comprising: a circulating pump coupled to urge the circulating liquid to move along a closed loop.
 7. The system of claim 6 further comprising: a control valve located between said flash tank and said heat exchanger to control the flow of blow down to said heat exchanger.
 8. The system of claim 7 further comprising: a temperature sensor coupled to selectively operate said control valve to control the flow of blow down to the heat exchanger as the temperature of the circulating liquid changes.
 9. The system of claim 8 wherein said control valve is a 3-way valve.
 10. The system of claim 9 wherein said 3-way valve prevents blow down from reaching said heat exchanger when the temperature of the circulating liquid reaches a predetermined temperature.
 11. The method of heating combustion air before the combustion air is received by a combustion air blower of a steam boiler comprising: coupling a flash tank to the steam boiler to receive blow down; coupling a heat exchanger to receive blow down from said flash tank and a circulating liquid to transfer heat from said blow down to the circulating liquid; and feeding said circulating liquid to a preheat coil to transfer heat from the circulating liquid to combustion air going to a combustion air blower.
 12. The method of claim 11 wherein said heat exchanger is a liquid-to-liquid heat exchanger.
 13. The method of claim 12 further comprising: providing a first damper to modulate the flow of combustion air from said preheat coil.
 14. The method of claim 13 wherein said first damper is located downstream of the combustion air blower.
 15. The method of claim 14 further comprising: positioning a second damper to modulate combustion air to the preheat coil.
 16. The method of claim 15 further comprising: using a circulating pump to move the circulating liquid along a closed loop.
 17. The method of claim 16 further comprising: locating a control valve between said flash tank and said heat exchanger to control the flow of blow down to said heat exchanger.
 18. The method of claim 17 further comprising: connecting a temperature sensor to selectively operate said control valve to control the flow of blow down to the heat exchanger as the temperature of the circulating fluid changes.
 19. The method of claim 18 wherein said control valve is a 3-way valve.
 20. The method of claim 19 wherein said 3-way valve prevents blow down from reaching said heat exchanger when the temperature of the circulating liquid reaches a predetermined temperature. 